Gas phase polymerization converts a gaseous monomer into granular solids. The gaseous monomer, and optional comonomer, are introduced under pressure into a reaction vessel, i.e. reactor, containing a catalyst and optional activator. Once polymerization begins, the monomer molecules diffuse to the growing polymer chains within the reactor. The resulting polymer is obtained as a granular solid which is fluidized within the reactor with the monomer and catalyst.
Gas phase polymerization is particularly prone to depositing solid particles on the reactor walls and other process exposed surfaces of the reactor due to static charge or electrical attraction between a metal surface and the polymer. Over time, the solids can accumulate and grow to form a solid sheet of polymer on the metal, such as a reactor wall or internal distribution plate, injection nozzle or product discharge nozzle. This phenomenon is common in the art and is known as “sheeting” when the polymer forms on a wall of the reactor, or “plugging” when the polymer forms on the distributor plate or nozzles and blocks the flow of fluid therethrough.
Polymer sheets on the walls of the reactor can grow in height and thickness to the point where the weight of the sheet overcomes any attractive forces between the sheet and the metal and falls to the bottom of the reactor. Solid polymer can also grow to an appreciable thickness to plug or block fluid flow paths on the nozzles or distribution plate. In either case, the solid polymer can plug or block monomer injection, catalyst injection, and/or product discharge. The solid polymer can also inhibit or interfere with fluidization within the reactor. As a result, the polymer product can become off spec and/or polymerization can come to a stop. To remove the solid polymer, the reactor is usually purged and shut down, which is both costly and time-consuming.
Certain pretreatment techniques have been used to prevent or control sheeting or plugging within the reactor. For example, a liquid catalyst has been sprayed onto the walls of the reactor and reacted with the monomer to produce a polymer coating or layer on the reactor wall that serves as an insulation layer to prevent product polymer growth thereon. The liquid catalyst is typically a chromocene. Such technique is described in more detail in U.S. Pat. Nos. 4,532,311, 4,792,592, and 4,876,320.
To be an effective tool for preventing or controlling sheeting or plugging within the reactor, the polymer should be even and consistently distributed throughout the reactor. The reaction time is estimated based on experience; however, due to varying conditions or other interruptions, the polymer growth is often uncertain and unpredictable. Thus, to assess the success of the chromocene treatment, the reactor is typically purged and opened to visually inspect the polymer coating and/or to measure the layer thickness. Often times, to the disappointment of the operator, no coating is found upon inspection and/or the coating is not evenly distributed.
There is a need, therefore, for new apparatus and methods for an on-line, real time detection of the polymer thickness during the chromocene treatment.